Clad aluminum alloy materials for brazing applications having an aluminum alloy core material and aluminum alloy filler material as a surface skin is well known. This material is most commonly formed by superimposing an aluminum alloy plate which serves as a skin material over an aluminum alloy cast ingot which serves as the core material. The filler plate can be applied to one or both surfaces of the ingot. The composite then undergoes hot rolling to bond the two layers together to thicknesses of about 3 to about 10 mm. The clad material is then subsequently cold rolled down to thicknesses of about 0.075 to about 3 mm to obtain brazing sheet. The sheet may be annealed to aid in rolling efficiency or to achieve desired final mechanical properties.
Aluminum brazing sheet is used in a variety of thicknesses, depending on the application. For instance, brazing sheet used in air conditioners have thicknesses ranging from about 1.5 to about 2 mm when being used for support purposes, from 0.5 to 0.65 mm as evaporator plates, from about 0.3 to about 0.4 mm as tube stock (sheet material formed into tubes for carrying fluids, i.e. gases, vapors, and liquids) and from about 0.075 to about 0.15 mm as fin stock (sheet material from which, for instance, radiator fins are made).
More recently, clad brazing sheet has been made by continuously casting the core alloy between a pair of rolls while laminating a sheet (band) of the cladding alloy to the core alloy at the rolls in a system described in U.S. Pat. No. 5,476,725, incorporated herein by reference.
Only certain clad aluminum sheet products have been produced by this method of continuously laminating a cladding band onto a casting core alloy, namely, aluminum brazing sheet. The alloy of the core of a brazing sheet typically is produced from an alloy of the 3XXX or 6XXX Aluminum Association (AA) series and the clad layer generally is an AA 4XXX series alloy. Consequently, the clad layer of aluminum brazing sheet conventionally has a lower solidus temperature (the temperature at which some melting of the material first occurs) than the solidus temperature of the core alloy.
The production of other clad aluminum sheet products, particularly bright aluminum sheet products, is accomplished via the conventional method of roll bonding a clad layer onto a cast ingot of a core alloy. Bright sheet products require that the clad layer be produced from a high purity aluminum alloy (at least 99.5 wt. % aluminum) which has a higher solidus temperature than the solidus temperature of the core. The roll bonded high purity aluminum clad layer may be further finished by electrobrightening and anodizing to achieve a desired reflective finish to the sheet. Bright products should have a minimum of surface defects (dents, scratches or spots) with a high degree of surface brightness and uniformity to ensure good and uniform optical properties of the sheet after anodizing. These properties have heretofore only been achieved by roll bonding the high purity aluminum clad layer to a cast ingot. The cladding ratio (the ratio of the thickness of the clad layer to the total sheet thickness) typically is about 5 to 20%. Uniformity in the cladding ratio along a roll bonded bright sheet in the transverse and longitudinal directions has previously been poor, particularly at higher cladding ratios. While cladding ratios of 5% can be produced with differences of 1% along a roll bonded bright sheet, the degree of nonuniformity increases with greater cladding ratios and reaches 3% nonuniformity for cladding ratios of 20%.
Hence, a need remains for a method of producing bright sheet products in a continuous process which meets the surface quality requirements for bright sheet product with high uniformity in cladding ratio.